Hybrid-type operating machines that utilize an internal combustion power source and an electric motor power source to achieve efficient operations have been developed and put into practical use. As hybrid-type operating machines, those employing a parallel-type drive configuration are known in the field.
In the parallel-type drive configuration, a hydraulic pump and a power machine serving to provide both a power generator function and an electric motor function are connected to an internal combustion engine serving as a shared power source. While the hydraulic pump drives a hydraulic actuator, the power machine exerts a power generator function to charge an electric storage device. Electric power from this electric storage device is used by the power machine to operate as an electric motor to assist the engine. The power machine may be a dual-purpose machine (i.e., a power generator and electric motor machine) that serves to provide both a power generator function and an electric motor function, or may be a power generator and an electric motor that are separately provided.
In such a hybrid-type operating machine, the load on the engine is reduced, and the engine is operated in a highly efficient operating range to achieve energy saving. However, related-art hybrid-type operating machines have problems as follows.
Charge-discharge characteristics of an electric storage device such as a battery (i.e., secondary battery) such as a lithium-ion battery or a capacitor (i.e., electric double layer capacitor) are dependent on the amount of electric charge. As the amount of electric charge decreases, the maximum charge electric power increases, and the maximum discharge electric power decreases. Since a power ratio between the engine and the power machine is determined without regard to the amount of electric charge in the electric storage device, the amount of electric charge may become too large or too small, depending on the condition of load. As a result, the capacity of the electric storage device may not be fully utilized, and, also, the electric storage device may deteriorate.
In order to solve the problems as described above, a power source apparatus for an operating machine has been developed that determines a power ratio between the engine and the power machine in response to the amount of electric charge in the electric storage device, thereby maintaining the amount of electric charge in the electric storage device in a proper range (see Patent Document 1, for example). In this power source apparatus, a hydraulic pump and a power-generator-and-electric-motor dual-purpose machine are connected in parallel to the engine serving as a shared power source. A battery provided as an electric storage device is charged by the power generator function of the power-generator-and-electric-motor dual-purpose machine. Further, power discharged from the battery drives the power-generator-and-electric-motor dual-purpose machine to exert an electric motor function. A power ratio between the engine and the power-generator-and-electric-motor dual-purpose machine is determined based on the power required by the actuator, a selected engine power, and the charge power and discharge power that are selected in response to the amount of battery charge in such a manner as to maintain the amount of battery power in a certain definite range.
[Patent Document 1] Japanese Patent Application Publication No. 2005-237178